You know, to give styling advice, I would first need to know what you envision or would want for this tree?
As with all (or just about all) young trees, the only way to get a trunk to thicken to a decent caliper is to let it grow, either in the ground or LARGE grow boxes relative to the plant size. Right now it looks tiny. Once you put it in a small bonsai pot, the trunk will stop thickening. I know from experience, I had a pomegranate that I kept in a bonsai pot, for forty years, yes, 40 years. At the end, the trunk was only 1 inch in diameter. That one inch diameter trunk was not thick enough to make a convincing shohin, much less a larger tree. But if the same pomegranate had been allowed to grow out, in a large grow pot, it easily could have been over 4 inches in diameter before it was 10 years old.
Lets assume you want a shohin tree about 8 inches tall. You will want to get the trunk up to about 1.5 to 2 inches in diameter at a minimum before moving from a large nursery pot to a bonsai pot. Best diameter would be about 2.4 inches for an ''old tree'' look on a shohin about 8 inches tall. This means you will have to let a leader get to at least 4 or 5 feet tall, maybe more at some point. It will need enough roots to fill a 3 gallon nursery pot. You have a few years of the ''nurseryman's activities'' phase of bonsai ahead of you for this tiny seedling. You may have to step it up from the pot it is in to a bigger pot, but wait on the move until the tree's roots begin to fill the current pot. Don't over pot, but keep stepping it into larger pots as it grows until you have the diameter trunk you need.
Fastigiate cultivars of most trees (the narrow columnar forms) generally are iffy choices for bonsai. When used, they are best for formal and informal uprights, seldom or never used for cascades or any wide spreading styles. If this were mine, I would go for a formal upright, as I don't have one. A good formal upright is difficult to do, and the best ones were done from young nursery plants rather than collected material. If you go formal upright, do not put any bends in the trunk, keep it arrow straight. The taper in a formal upright is built by successive trunk chops, building the tree one segment at a time. First chop is done when the trunk reaches pretty much most of the desired diameter for the finished tree, then at about 25% to 40% the final desired height the first chop is done, and a new leader is trained. When the diameter of the second section reaches about 60 to 75% the diameter of the first segment the second chop is made at a point less than the length of the first segment. repeat for the third segment, with the length of the third segment of trunk being less than the length of the second. the new leader above the third segment should become the apex of the final tree.
For an informal upright, pretty much the same plan is followed, just add curves within segments, and direction changes at each chop between segments. Build a Tree 101.
As a general guideline branches should always be 50% less in diameter of the trunk. Give or take, if a branch gets too close to the diameter of a trunk, it ruins the sense of perspective, making the image more shrub like than tree like. This means that most if not all branches that currently exist on the tree will be removed and replaced with smaller ones as the tree approaches the right diameter to be a bonsai. With yews, because they bud back on old wood quite well, just leat the branches run wild until your trunk is over 2 inches in diameter at the base. Then chop them all off and start over with the new buds that sprout along the trunk.
Well, my proposal, if it were mine, pretty much covers the next 5 years, or more. When it gets there, post pictures and I'd be happy to let you know what I think.
Read the articles on Brent Walston's website about building a bonsai. His articles are geared for the ''nurseryman's activities'' with young material.
http://www.evergreengardenworks.com/articles.htm